Semiconductor device and method for producing the same

ABSTRACT

A semiconductor laser device including: a semiconductor substrate of a first conductivity type; a cladding layer of the first conductivity type provided on the semiconductor substrate; an active layer provided on the cladding layer of the first conductivity type, the active layer having a super-lattice structure including a disordered region in a vicinity of at least one cavity end face; a first cladding layer of a second conductivity type provided on the active layer; an etching stop layer of the second conductivity type provided on the first cladding layer; and a second cladding layer of the second conductivity type provided on the etching stop layer, the second cladding layer forming a ridge structure, the ridge structure extending along a cavity length direction and having a predetermined width. A concentration of an impurity in the etching stop layer in the vicinity of the at least one cavity end face is greater than a concentration of the impurity in the interior of a cavity and equal to or smaller than about 2×10 18  cm −3 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor laser device, and amethod for producing the same. In particular, the present inventionrelates to a semiconductor laser device having a window structure forproviding an improved production yield, and a method for producing thesame.

2. Description of the Related Art

In recent years, semiconductor laser devices for use as light sourcesfor information processing apparatuses (e.g., optical disk apparatuses)have been expected to provide higher and higher output power in order toattain higher recording speeds. One method for satisfying such needsinvolves the use of a window structure at an end face of a laser cavity,which improves the catastrophic optical damage (hereinafter “COD”) levelat the end face.

COD is an instantaneous degradation phenomenon which occurs as theoptical output of a semiconductor laser is increased to or above acertain limit value. A COD phenomenon occurs in the case where thevicinity of an end face of a semiconductor laser becomes an absorptionregion with respect to the light which occurs within the laser.

Specifically, a COD phenomenon may occur when dangling bonds are formeddue to oxygen adsorption or surface oxidation on the semiconductorsurface of the end face of the laser cavity, generating a peculiar deeplevel on the semiconductor surface, thereby substantially narrowing theforbidden band width in the vicinity of the end face. Since anynon-radiative recombination due to a surface level present on thesemiconductor surface introduces an increase in temperature, theoccurrence of a COD phenomenon results in further reduction of theforbidden band width in the vicinity of the end faces of the lasercavity and further increases light absorption. Thus, there is a positivefeedback with respect to the COD phenomenon. As a result, the end facesmay be destroyed due to melting or the like, which leads to a decreasein the optical output as well as irreversible degradation of the device.

Some semiconductor laser devices employ a window structure so as toenhance the band gap energy in a portion of an active layer near a lasercavity end face, thereby preventing COD destruction at the cavity endface. For example, a window structure can be realized by diffusing animpurity in the vicinity of a laser cavity end face so as to disorderthe super-lattice structure in the active layer.

Hereinafter, a method for producing a conventional window structuresemiconductor laser device will be described. FIGS. 6A to 6D arediagrams illustrating respective steps of a manufacturing process for aconventional window structure semiconductor laser device.

First, as shown in FIG. 6A, the following semiconductor multilayerstructure is formed on an n-GaAs substrate 601 by using an MOVPE (metalorganic vapor phase epitaxy) method, where the respective layers aresequentially grown into crystals in this order: an n-AlGaInP claddinglayer 602, an AlGaInP/GaInP super-lattice active layer 603, a p-AlGaInPfirst cladding layer 604, a p-GaInP etching stop layer 605, a p-AlGaInPsecond cladding layer 606, a p-GaInP band graded layer 607, and a p-GaAscapping layer 608.

Next, as shown in FIG. 6B, by using a plasma CVD (chemical vapordeposition) technique, an SiN film 609 is formed on the aforementionedsemiconductor multilayer structure. Furthermore, the SiN film 609 ispatterned by dry etching so as to form two parallel openings whoseplanar forms appear as two stripes having a width of several dozen μm. Awet etching step removes the portions of the GaAs capping layer 608where these openings are formed. Thereafter, a ZnO film 610 and an SiO₂film 611 are formed by sputtering so as to cover the semiconductormultilayer structure (including the openings). Furthermore, an annealingis performed so as to diffuse Zn from the ZnO film 610 through theportions of the p-GaInP layer 607 which are exposed in the openings ofthe SiN film 609, and the openings in the GaAs capping layer 608.Through such solid-phase diffusion of Zn, impurity diffusion regions 612having stripe-like planar forms are formed, and the portions of theAlGaInP/GaInP super-lattice active layer 603 which lie within theimpurity diffusion regions 612 are converted into a mixed crystal. Theregions of the active layer 603 which have been converted into the mixedcrystal define window structures. The window structures have a higherband gap energy than that of the regions which have not formed a mixedcrystal.

Referring to FIG. 6C, the SiO₂ film 611, the ZnO film 610, the SiN film609, and the GaAs film 608 are removed. Thereafter, by using a knowntechnique, a stripe pattern of SiO₂ film 613 is formed on the exposedp-GaInP band graded layer 607 so as to extend along a plane which isperpendicular to the longitudinal direction of the impurity diffusionregions 612. By using the stripe pattern of SiO₂ film 613 as a mask, thep-GaInP band graded layer 607 is etched into a ridge shape by using anacetic acid-type etchant. Then, switching to a sulfuric acid-typeetchant, the p-AlGaInP second cladding layer 606 is etched away untilreaching the p-GaInP etching stop layer 605. As a result, a ridgestructure composed of the p-GaInP band graded layer 607 and thep-AlGaInP second cladding layer 606 is formed as shown in FIG. 6C. Sincethe sulfuric acid-type etchant has a greater etching rate for thep-AlGaInP second cladding layer 606 than for the p-GaInP etching stoplayer 605, the etching process can be successfully stopped at theetching stop layer 605.

Thereafter, an n-type current blocking layer 614 is grown so as to burythe side of the ridge structure by a selective growth technique using anMOVPE method. After removing the SiO₂ film 613 serving as a stripe mask,a p-GaAs contact layer 615 is grown over the n-type current blockinglayer 614 and the p-GaInP band graded layer 607. By using a knowntechnique, p-side and n-side ohmic electrodes are formed (not shown).

The resultant semiconductor multilayer structure is cleaved in theimpurity diffusion regions 612 along a plane perpendicular to thelongitudinal direction of the ridge structure, thereby forming lasercavity end faces. As a result, a semiconductor laser device havingwindow structures as shown in FIG. 6D is accomplished.

Conventional semiconductor laser devices with window structures areformed by the above-described manufacturing process. However, inaccordance with the above-described manufacturing process, not only theactive layer 603 but also the p-GaInP etching stop layer 605 areconverted into a mixed crystal together with the surrounding AlGaInPlayers during the step of Zn diffusion. That is, in accordance withabove-described conventional manufacturing process, Zn is directlydiffused from the Zn source, i.e., the ZnO film 610, into the AlGaInPlayers, which have a relatively large diffusion coefficient. Therefore,it is difficult to control the impurity dose amount. As a result, asshown in FIG. 1, for example, a high concentration of impurity isdiffused in the AlGaInP crystal, allowing for a rapid development of themixed crystal. In particular, the thin etching stop layer 605 mayeventually be destroyed by the etching. In that case, since the etchingselection ratio of the sulfuric acid-type etchant is extremelydecreased, the etching cannot be stopped by the etching stop layer 605,allowing the p-AlGaInP first cladding layer 604 and the active layer 603to be etched. Thus, the ridge shape may not be controlled properly.

Moreover, in accordance with the above-described conventionalmanufacturing process, the current blocking layer 614 may be formed soas to be nearer the active layer 603 due to overetching. As a result,the angle of expanse of light exiting the active layer 603 cannot beeffectively controlled. In a loss-guide type semiconductor laser device,in particular, the propagation loss is increased so that the lasercharacteristics of the device are greatly deteriorated, resulting in,e.g., a decrease in the output power, or an increase in the operationcurrent.

Furthermore, in accordance with the above-described conventionalmanufacturing process, the impurity concentration in the active layer603 becomes very high as shown in FIG. 1. As a result, the propagationloss due to carrier scattering is increased so that the lasercharacteristics of the device are greatly deteriorated, resulting in,e.g., a decrease in the output power, or an increase in the operationcurrent.

As a method for avoiding overetching, for example, Japanese Laid-OpenPublication No. 9-139550 discloses a method which involves firstexposing the AlGaInP layer 606 by removing the p-GaInP layer 607 whileleaving portions of the p-GaInP layer 607 only in regions where Zn hasbeen diffused, and then performing an etching with a sulfuric acid-typeetchant. According to this method, at the point in time where theetching has reached the etching stop layer 605 in regions other than theZn diffusion regions, the AlGaInP layer 606 is left in the Zn diffusionregions due to the low etching rate for the p-GaInP band graded layer607, and the etching is terminated at this point.

However, it is evidently difficult to accurately control the etchingamount for the AlGaInP layer 606. The remainder of the AlGaInP layer 606after etching varies per every etching.

Therefore, the height of the ridge which is formed as a result of theetching may vary, thus making it difficult to control the ridge shape.

SUMMARY OF THE INVENTION

A semiconductor laser device according to the present inventionincludes: a semiconductor substrate of a first conductivity type; acladding layer of the first conductivity type provided on thesemiconductor substrate; an active layer provided on the cladding layerof the first conductivity type, the active layer having a super-latticestructure including a disordered region in a vicinity of at least onecavity end face; a first cladding layer of a second conductivity typeprovided on the active layer; an etching stop layer of the secondconductivity type provided on the first cladding layer; and a secondcladding layer of the second conductivity type provided on the etchingstop layer, the second cladding layer forming a ridge structure, theridge structure extending along a cavity length direction and having apredetermined width, wherein a concentration of an impurity in theetching stop layer in the vicinity of the at least one cavity end faceis greater than a concentration of the impurity in the interior of acavity and equal to or smaller than about 2×10¹⁸ cm⁻³.

In one embodiment of the invention, the semiconductor substratecomprises a compound semiconductor material containing GaAs of the firstconductivity type as a main component; the cladding layer of the firstconductivity type comprises a compound semiconductor material containingGaP of the first conductivity type as a main component; and the activelayer comprises a compound semiconductor material containing GaP as amain component, and the first cladding layer, the etching stop layer,and the second cladding layer each comprise a compound semiconductormaterial containing GaP of the second conductivity type as a maincomponent.

In another embodiment of the invention, the semiconductor substratecomprises GaAs of the first conductivity type; the cladding layer of thefirst conductivity type comprises AlGaInP of the first conductivitytype; the active layer comprises AlGaInP and GaInP: the first claddinglayer comprises AlGaInP of the second conductivity type; the etchingstop layer comprises GaInP of the second conductivity type; and thesecond cladding layer comprises AlGaInP of the second conductivity type.

In still another embodiment of the invention, a concentration gradientof the impurity in the second cladding layer in the vicinity of the atleast one cavity end face, taken along a normal direction to thesubstrate from an upper face toward a bottom face of the substrate, isgreater than a concentration gradient of the impurity in the interior ofthe cavity along the normal direction to the substrate, and is equal toor smaller than about 2×10¹⁸ cm⁻³ μm⁻¹.

In still another embodiment of the invention, a concentration of theimpurity in the active layer in the vicinity of the at least one cavityend face is greater than a concentration of the impurity in the interiorof the cavity, and is equal to or smaller than about 2×10¹⁸ cm⁻³.

In still another embodiment of the invention, the impurity is Zn.

In another aspect of the invention, there is provided a method forproducing a semiconductor laser device including the steps of: forming asemiconductor multilayer structure on a semiconductor substrate of afirst conductivity type, the semiconductor multilayer structureincluding: a cladding layer of the first conductivity type; an activelayer having a super-lattice structure; a first cladding layer of asecond conductivity type; an etching stop layer of the secondconductivity type; a second cladding layer of the second conductivitytype; a band graded layer of the second conductivity type; and animpurity supply control layer; disordering the active layer by diffusingan impurity at least in a predetermined region within the semiconductormultilayer structure; and patterning the second cladding layer into aridge structure by wet etching, wherein a concentration of the impuritydiffused in the etching stop layer within the predetermined region isgreater than a concentration of the impurity outside the predeterminedregion and equal to or smaller than about 2×10¹⁸ cm⁻³.

In one embodiment of the invention, the semiconductor substratecomprises a compound semiconductor material containing GaAs of the firstconductivity type as a main component; the cladding layer of the firstconductivity type comprises a compound semiconductor material containingGaP of the first conductivity type as a main component; the active layercomprises a compound semiconductor material containing GaP as a maincomponent; the first cladding layer, the etching stop layer, the secondcladding layer, and the band graded layer each comprise a compoundsemiconductor material containing GaP of the second conductivity type asa main component; and the impurity supply control layer comprises acompound semiconductor material containing GaAs as a main component.

In another embodiment of the invention, the semiconductor substratecomprises GaAs of the first conductivity type; the cladding layer of thefirst conductivity type comprises AlGaInP of the first conductivitytype; the active layer includes a super-lattice structure comprisingAlGaInP and GaInP; the first cladding layer and the second claddinglayer each comprise AlGaInP of the second conductivity type; the etchingstop layer comprises GaInP of the second conductivity type; the bandgraded layer comprises GaInP of the second conductivity type; and theimpurity supply control layer comprises GaAs.

In still another embodiment of the invention, the impurity supplycontrol layer has a thickness equal to or greater than about 100 Å.

In still another embodiment of the invention, a concentration gradientof the impurity diffused in the second cladding layer within thepredetermined region, taken along a normal direction to the substratefrom an upper face toward a bottom face of the substrate, is greaterthan a concentration gradient of the impurity outside the predeterminedregion along the normal direction to the substrate, and is equal to orsmaller than about 2×10¹⁸ cm⁻³ μm⁻¹.

In still another embodiment of the invention, a concentration of theimpurity diffused in the active layer within the predetermined region isgreater than a concentration of the impurity outside the predeterminedregion, and is equal to or smaller than about 2×10¹⁸ cm⁻³.

In still another embodiment of the invention, the impurity is Zn.

Thus, the invention described herein makes possible the advantages of(1) providing a semiconductor laser device having window structureswhich can be manufactured with a good production yield; and (2)providing a method for producing such a semiconductor laser device.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram which illustrates Zn concentration in various layersof a window structure semiconductor laser device according to thepresent invention and a conventional window structure semiconductorlaser device.

FIGS. 2A to 2D are perspective views illustrating respective steps of amanufacturing process for a window structure semiconductor laser deviceaccording to the present invention.

FIG. 3 is a graph obtained by measuring evaluation samples, showing a Zndiffusion profile illustrating the diffusion of Zn into GaAs duringsolid-phase diffusion.

FIG. 4 is a graph illustrating the relationship between theconcentration of Zn which is diffused within a GaInP etching stop layerand overetching depth according to a step in the manufacturing processof the present invention.

FIG. 5 is a graph illustrating results of a reliability test withrespect to a plurality of semiconductor laser samples having differentconcentration levels of Zn which is diffused within the GaInP etchingstop layer.

FIGS. 6A to 6D are diagrams illustrating respective steps of amanufacturing process for a conventional window structure semiconductorlaser device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a method for producing a semiconductor laser deviceaccording to the present invention will be described. FIGS. 2A to 2D arediagrams illustrating respective steps of a manufacturing process for awindow structure semiconductor laser device according to the presentinvention.

First, as shown in FIG. 2A, the following semiconductor multilayerstructure is formed on an n-GaAs substrate 201 by using an MOVPE method,where the respective layers are sequentially grown into crystals in thisorder: an n-AlGaInP cladding layer 202, an AlGaInP/GaInP super-latticeactive layer 203, a p-AlGaInP first cladding layer 204, a p-GaInPetching stop layer 205, a p-AlGaInP second cladding layer 206, a p-GaInPband graded layer 207, and a p-GaAs impurity supply control layer 208.As the crystal growth method, any other method may be used, e.g., an MBE(molecular beam epitaxy) method.

Next, as shown in FIG. 2B, a ZnO film 209 is formed on theaforementioned semiconductor multilayer structure by sputtering. Afteran organic resist film (not shown) is applied on the ZnO film 209,stripes having a width of several dozen μm are formed by usingphotolithography techniques. The portions of the ZnO film 209 and thep-GaAs impurity supply control layer 208 which lie outside the stripesare removed by wet etching. However, such portions of the p-GaAsimpurity supply control layer 208 are not removed all the way down tothe underlying p-GaInP band graded layer 207. Thereafter, by using aplasma CVD technique, an SiN film 210 is formed on the semiconductormultilayer structure so as to overlay the stripes. Furthermore, anannealing is performed so as to diffuse Zn from the ZnO film 209.Specifically, the Zn is solid-phase diffused in a direction from theupper face of the p-GaAs impurity supply control layer 208 toward thesubstrate 201, so as to reach the n-AlGaInP cladding layer 202. Throughsuch solid-phase diffusion of Zn, stripe-like impurity diffusion regions211 are formed so as to be parallel to each other. The impuritydiffusion regions 211 will be located in the vicinity of the cavity endfaces of a completed semiconductor laser device so as to function aswindow structures of the semiconductor laser device.

In the stripe-like impurity diffusion regions 211 which are formedthrough the aforementioned solid-phase diffusion of Zn, not only thecrystal of the active layer 203 but also the crystal of the etching stoplayer 205 are disordered. Window structures are provided through thedisordering of the active layer 203. However, the disordering actionalso results in the decrease of the function of the etching stop layer205. Stated otherwise, when producing window structures, it is ideal todisorder the active layer 203 while conserving the function of theetching stop layer 205. To this end, it is important to perform thesolid-phase diffusion of Zn (or more generally, any impurity that isdiffused) in such a manner that the concentration of Zn which isdiffused into the respective layers of the semiconductor multilayerstructure, in particular the n-AlGaInP cladding layer 202, theAlGaInP/GaInP super-lattice active layer 203, the p-AlGaInP firstcladding layer 204, the p-GaInP etching stop layer 205, and thep-AlGaInP second cladding layer 206, is at a low level.

Hereinafter, a method according to the present invention will bedescribed which controls the diffusion concentration of Zn (or moregenerally, any impurity that is diffused) within the stripe-likeimpurity diffusion regions 211 in the semiconductor multilayerstructure.

FIG. 3 is a graph showing a diffusion profile of Zn, obtained bymeasuring evaluation samples. Specifically, each evaluation sample wasobtained by forming an SiN capping layer on a ZnO film, which in turnwas deposited on a GaAs layer, and the evaluation sample was heated in anitrogen atmosphere so as to cause solid-phase diffusion of Zn. As canbe seen from this graph, the diffusion behavior of Zn within the GaAslayer is such that both a low-concentration diffusion front (Znconcentration: about 2×10¹⁸ cm⁻³) having a relatively high diffusionspeed and a high-concentration diffusion front (Zn concentration: about3×10¹⁹ cm⁻³) having a relatively low diffusion speed are present.Moreover, in general, a compound semiconductor material containing P asa V group element, e.g., AlGaInP and GaInP, has a much higher Zndiffusion speed than that of a compound semiconductor materialcontaining As as a V group element, e.g., AlGaAs and GaAs.

By taking advantage of the aforementioned diffusion behavior of Zn, thefollowing procedure is employed according to the present invention inorder to control to a low level the Zn concentration in the respectiveAlGaInP layers which are located between the p-GaInP band graded layer207 and the substrate 201, i.e., the n-AlGaInP cladding layer 202, theAlGaInP/GaInP super-lattice active layer 203, the p-AlGaInP firstcladding layer 204, the p-GaInP etching stop layer 205, and thep-AlGaInP second cladding layer 206, within the stripe-like impuritydiffusion regions 211 in the semiconductor multilayer structure.

First, the ZnO film 209 is formed on the p-GaAs impurity supply controllayer 208 in the aforementioned manner so that the p-GaAs impuritysupply control layer 208 is located between the ZnO film 209 and thep-GaInP band graded layer 207. Furthermore, the diffusion conditions areprescribed so that only the low-concentration Zn diffusion front in GaAsreaches the interface between the p-GaAs impurity supply control layer208 and the p-GaInP band graded layer 207. As a result, the Znconcentration within the stripe-like impurity diffusion regions 211 ofthe respective layers of the semiconductor multilayer structure whichare located between the p-GaInP band graded layer 207 and the substrate201 can be controlled to a low level (i.e., about 2×10¹⁸ cm⁻³ or less).

FIG. 1 is a diagram which illustrates Zn concentration in various layersof the semiconductor laser device according to the present invention anda conventional window structure semiconductor laser device. It will beseen from FIG. 1 that the Zn concentration in the p-GaInP etching stoplayer 205 is about 2×10¹⁸ cm⁻³ or less. In this case, the etching stoplayer 205 is prevented from entering the state of being an undesirablyhigh degree of mixed crystal.

As seen from the results according to the present invention shown inFIG. 1, the diffusion concentration of the impurity (i.e., Zn in thisexample) is about 2×10¹⁸ cm⁻³ or less not only in the p-GaInP etchingstop layer 205 but also in the AlGaInP/GaInP super-lattice active layer203, the p-AlGaInP first cladding layer 204, and the p-AlGaInP secondcladding layer 206. The impurity (Zn) diffusion concentration is evenlower in the layers located between the active layer 203 and thesubstrate 201 (e.g., the portion corresponding to the n-AlGaInP claddinglayer 202).

It should also be noted that the aforementioned value of 2×10¹⁸ cm⁻³ ishigher than the impurity concentration in regions outside the impuritydiffusion regions 211, i.e., the interior of the cavity of thesemiconductor laser device, (also shown in FIG. 1).

The p-GaAs impurity supply control layer 208 preferably has a thicknessof about 100 Å or more. If the p-GaAs impurity supply control layer 208is less than about 100 Å thick, it is difficult to controllably stop thehigh-concentration Zn diffusion front in the p-GaAs impurity supplycontrol layer 208.

Referring to FIG. 2C, the SiN film 210 and the ZnO film 209 are removedby using an appropriate etchant, e.g., a hydrofluoric acid. Furthermore,by using a mixed solution of sulfuric acid and hydrogen peroxide, thep-GaAs impurity supply control layer 208 is etched away. Thereafter, byusing a known technique, a stripe pattern of SiO₂ film 212 is formed onthe exposed p-GaInP band graded layer 207 so as to extend along a planewhich is perpendicular to the longitudinal direction of the impuritydiffusion regions 211. By using the stripe pattern of SiO₂ film 212 as amask, the p-GaInP band graded layer 207 is etched into a ridge shape byusing an acetic acid-type etchant. Then, using a sulfuric acid-typeetchant, the p-AlGaInP second cladding layer 206 is etched away untilreaching the p-GaInP etching stop layer 205. As a result, a ridgestructure composed of the p-GaInP band graded layer 207 and thep-AlGaInP second cladding layer 206 is formed as shown in FIG. 2C.

Since the sulfuric acid-type etchant has a greater etching selectionrate for the p-AlGaInP second cladding layer 206 than for the p-GaInPetching stop layer 205 in regions other than the stripe-like impuritydiffusion regions 211 (which correspond to the interior of the cavity ofa completed semiconductor laser), the etching process can besuccessfully stopped at the etching stop layer 205. On the other hand,in the stripe-like impurity diffusion regions 211 (which correspond tothe window structures of a completed semiconductor laser), Al isdiffused into the GaInP because the GaInP and the AlGaInP have formed amixed crystal. Therefore, the portions of the etching stop layer 205which lie in the stripe-like impurity diffusion regions 211 have ahigher etching rate than that for portions other than the stripe-likeimpurity diffusion regions 211.

However, the degree of mutual diffusion depends on the amount of Znwhich is implanted through diffusion. By reducing the Zn concentrationin the etching stop layer 205 within the stripe-like impurity diffusionregions 211 using the aforementioned method according to the presentinvention to about 2×10¹⁸ cm⁻³ or less, it is possible to retain a highetching selection ratio for the p-AlGaInP second cladding layer 206.

Thus, according to the present invention, the etching for forming theridge structure is successfully stopped by the etching stop layer 205within the impurity diffusion regions 211 as well as in regions outsidethe impurity diffusion regions 211 (which correspond to the interior ofthe cavity of a completed semiconductor laser). As a result, the shapeand height of the resultant ridge are well controlled.

FIG. 4 is a graph illustrating the relationship between theconcentration of Zn which is diffused within the GaInP etching stoplayer 205 and overetching depth in the case where the GaInP etching stoplayer 205 fails to stop the etching so that the etching proceeds towardthe substrate 201.

As seen from FIG. 4, the overetching depth increases as the Znconcentration in the GaInP etching stop layer 205 increases. In otherwords, as the Zn concentration in the GaInP etching stop layer 205increases, the etching selection ratio between the second cladding layer206 and the etching stop layer 205 decreases due to the formation of amixed crystal in the second cladding layer 206 and the etching stoplayer 205, so that the etching cannot be properly stopped by the etchingstop layer 205. In contrast, by ensuring that the Zn concentrationwithin the impurity diffusion regions 211 of the etching stop layer 205are about 2×10¹⁸ cm⁻³ or less according to the present invention, itbecomes possible to prevent the overetching from occurring within theimpurity diffusion regions 211.

Referring back to FIG. 2C, portions of the SiO₂ film 212 whichcorrespond to the impurity diffusion regions 211 are removed byphotolithography and etching techniques so as to leave openings therein.

Then, as shown in FIG. 2D, an n-type current blocking layer 213 is grownso as to bury the ridge structure by a selective growth technique usingan MOVPE method. After removing the SiO₂ film 212 serving as a mask, ap-GaAs contact layer 214 is grown on the n-type current blocking layer213. By using a known technique, p-side and n-side ohmic electrodes areformed (not shown).

The resultant semiconductor multilayer structure is cleaved in theimpurity diffusion regions 211 along a plane perpendicular to thelongitudinal direction of the ridge structure, thereby forming lasercavity end faces. As a result, a semiconductor laser device havingwindow structures as shown in FIG. 2D is accomplished.

In order to study the effects of the present invention, a plurality ofsample semiconductor laser devices having different levels of Znconcentration diffused in the GaInP etching stop layer 205 wereproduced, and reliability tests were performed for the samples. Theresults are shown in FIG. 5. The reliability tests were performed underthe following conditions: the operation temperature was 60° C.; theoutput power was 35 mW; and the oscillation wavelength was 659 nm.

As seen from FIG. 5, overetching occurs in a semiconductor laser samplewhose Zn concentration in the GaInP etching stop layer 205 is 3×10¹⁸cm⁻³ for the aforementioned reason, and the propagation loss within thecurrent blocking layer 213 increases. As a result, rapid degradation(i.e., increase in the operation current) occurs under high poweroperation conditions due to light absorption in the vicinity of thecavity end faces. Thus, such a semiconductor laser cannot attainsufficient reliability. On the other hand, in a semiconductor lasersample according to the present invention, in which the Zn concentrationin the GaInP etching stop layer 205 is controlled to about 2×10¹⁸ cm⁻³or less, the ridge in the vicinity of end faces is adequately formed.Thus, the semiconductor laser according to the present inventionexhibits excellent reliability characteristics with a stable operationcurrent over long periods of time, so that a practicable high powerlaser is realized according to the present invention.

According to the present invention, the laser outgoing end face of thecompleted semiconductor laser is disordered due to Zn diffusion. As aresult, the active layer 203 in the disordered region near the laseroutgoing end face has a band gap which is greater than the band gap ofnon-disordered regions of the active layer 203. Thus, the disorderedregions of the active layer 203 define window structures. Since theconcentration of Zn which is diffused in the window structures iscontrolled at a low level, the etching stop layer 205 is prevented frombeing destroyed, so that overetching is minimized. As a result, theridge of the semiconductor laser device according to the presentinvention can be formed with good controllability.

Since the ridge of the semiconductor laser device according to thepresent invention is formed with good controllability, the propagationloss by the current blocking layer 213 can be prevented. Thus, ahigh-power semiconductor laser can be realized based on the preventionof propagation losses in the vicinity of cavity end faces.

According to the present invention, the concentration of Zn which isimplanted into portions of the respective layers of the semiconductormultilayer structure which are within the stripe-like impurity diffusionregions 211 is controlled to a low level (i.e., about 2×10¹⁸ cm⁻³ orless). In particular, the propagation loss due to carrier scattering inthe active layer 203 can be minimized.

Although the impurity supply control layer 208 is illustrated as beingcomposed of GaAs in the above example, the present invention is alsoapplicable to the case where the impurity supply control layer 208 iscomposed of another material types having a lower diffusion speed thanthat of an AlGaInP group material. In such cases as well, the sameeffects as those provided by the aforementioned GaAs impurity supplycontrol layer 208 can be achieved.

Compound semiconductor materials whose main components are GaP can beused as materials for constructing the respective layers of thesemiconductor multilayer structure.

Although the above example illustrates the case where the diffusedimpurity is Zn, the present invention is also applicable to any otherimpurity, e.g., Si. In such cases as well, the same effects as thoseprovided by the Zn impurity can be achieved.

The results obtained according to the present invention, shown in FIG.1, indicate that there is a diffusion concentration gradient of impurity(e.g., Zn) which is equal to or smaller than about 2×10¹⁸ cm⁻³ per μmalong the vertical direction (for conciseness, such a diffusionconcentration gradient will be referred to as “2×10¹⁸ cm⁻³ μm⁻¹ orless”) in the AlGaInP/GaInP super-lattice active layer 203, thep-AlGaInP first cladding layer 204, the p-GaInP etching stop layer 205,and the p-AlGaInP second cladding layer 206. Specifically, the diffusionconcentration gradient of impurity is taken along a direction which isperpendicular to the surface of the substrate 201 (hereinafter referredto as the “normal direction to the substrate), from the upper facetoward the bottom face of the substrate 201.

According to the present invention, the GaAs impurity supply controllayer 208 is effectively employed in conjunction with the disorderingprocess for the active layer 203. As a result, the concentration of animpurity (e.g., Zn) which is implanted into predetermined regions (i.e.,impurity diffusion regions) of the respective layers of thesemiconductor multilayer structure can be controlled to a low level.Thus, a wet etching performed for forming a ridge of the semiconductorlaser device can be successfully stopped by the etching stop layer,whereby the shape and height of the ridge can be accurately controlled.

Since the ridge of the semiconductor laser device according to thepresent invention can be formed with good controllability, it ispossible to control the expanse of light exiting the laser device. Sincethe current blocking layer 213 is prevented from being formed in theclose vicinity of the active layer 203 due to unwanted overetching, thepropagation loss is prevented.

According to the present invention, the concentration of an impurity(e.g., Zn) which is implanted and diffused into the crystals of therespective layers of the semiconductor multilayer structure iscontrolled to a low level (i.e., about 2×10¹⁸ cm⁻³ or less) by the useof the GaAs impurity supply control layer 208 in conjunction with thedisordering process for the active layer 203. Since the current blockinglayer 213 is prevented from being formed in the close vicinity of theactive layer 203 due to unwanted overetching, the propagation loss dueto carrier scattering in the active layer 203,in particular, can beminimized.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A semiconductor laser device comprising: asemiconductor substrate of a first conductivity type; a cladding layerof the first conductivity type provided on the semiconductor substrate;an active layer provided on the cladding layer of the first conductivitytype, the active layer having a super-lattice structure including adisordered region in a vicinity of at least one cavity end face; a firstcladding layer of a second conductivity type provided on the activelayer; an etching stop layer of the second conductivity type provided onthe first cladding layer; and a second cladding layer of the secondconductivity type provided on the etching stop layer, the secondcladding layer forming a ridge structure, the ridge structure extendingalong a cavity length direction and having a predetermined width,wherein a concentration of an impurity in the etching stop layer in thevicinity of the at least one cavity end face is greater than aconcentration of the impurity in the interior of a cavity and equal toor smaller than about 2×10¹⁸ cm⁻³, and the etching stop layer is asingle layer.
 2. A semiconductor laser device according to claim 1,wherein the semiconductor substrate comprises a compound semiconductormaterial containing GaAs of the first conductivity type as a maincomponent; the cladding layer of the first conductivity type comprises acompound semiconductor material containing GaP of the first conductivitytype as a main component; and the active layer comprises a compoundsemiconductor material containing GaP as a main component, and the firstcladding layer, the etching stop layer, and the second cladding layereach comprise a compound semiconductor material containing GaP of thesecond conductivity type as a main component.
 3. A semiconductor laserdevice according to claim 2, wherein the impurity is Zn.
 4. Asemiconductor laser device according to claim 1, wherein thesemiconductor substrate comprises GaAs of the first conductivity type;the cladding layer of the first conductivity type comprises AlGaInP ofthe first conductivity type; the active layer comprises AlGaInP andGaInP; the first cladding layer comprises AlGaInP of the secondconductivity type; the etching stop layer comprises GaInP of the secondconductivity type; and the second cladding layer comprises AlGaInP ofthe second conductivity type.
 5. A semiconductor laser device accordingto claim 1, wherein a concentration gradient of an impurity in thesecond cladding layer in the vicinity of the at least one cavity endface, taken along a normal direction to the substrate from an upper facetoward a bottom face of the substrate, is greater than a concentrationgradient of the impurity in the interior of the cavity along the normaldirection to the substrate, and is equal to or smaller than about 2×10¹⁸cm⁻³ μm⁻¹.
 6. A semiconductor laser device according to claim 1, whereina concentration of an impurity in the active layer in the vicinity ofthe at least one cavity end face is greater than a concentration of theimpurity in the interior of the cavity, and is equal to or smaller thanabout 2×10¹⁸ cm⁻³.